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Neutrino Astrophysics: A Research Briefing Why Neutrino Astrophysics? In the 1920s and 1930s, scientists developed the theoretical basis for understanding how the sun shines. They proposed  that nuclear fusion reactions among light elements occur near the center of the sun and provide the energy that has been emitted by the sun for 4.5 billion years. While most of this energy ends up as electromagnetic radiation from the surface, approximately 3 percent is believed to be emitted directly from the center of the sun in the form of neutrinos . In a pioneering experiment that has been going on for 25 years , Raymond Davis, Jr., and his collaborators first detected solar neutrinos using radiochemical techniques and a cleaning fluid (perchloroethylene) as a target. Over the last several years there has been striking confirmation of the astronomical predictions of solar neutrinos. The Kamiokande water Cerenkov detector located in the Japanese alps has identified solar neutrinos by their directionality, measured by observing the direction of recoil electrons from neutrino interactions [4,5]. Recently, two radiochemical experiments using gallium [6-9] (the GALLEX experiment in Italy and the SAGE experiment in Russia) have detected the copious low-energy (below 400 keV) neutrinos that are the primary component of the solar neutrino flux. A supernova represents one of the most awesome events in astronomy. Type II supernovae are believed to result from the sudden gravitational collapse of a star that has completed cycles of nuclear burning. The energy calculated to be produced from this collapse is almost 1000 times greater than that observed as light. Theory indicates that more than 99 percent of the energy is emitted in the form of neutrinos . In February 1987, neutrinos from SN1987A were detected as a series of pulses in two water Cerenkov detectors (Kamiokande in Japan  and IMB  in the United States), the neutrinos arriving, as expected, a few hours before the light from the explosion. The definitive detection of solar neutrinos and the detection of neutrinos from SN1987A are two of the most remarkable scientific discoveries of the last decade. They provide dramatic confirmation of fundamental theories concerning stellar interiors. The detection of solar neutrinos demonstrates that fusion energy is the basic source of energy received from the sun. The observations of solar and supernova neutrinos open up a new area of science: neutrino astrophysics. The panel notes that U.S. scientists have collaborated with their colleagues from different countries in each of these experiments. Neutrino astrophysics is naturally an international enterprise. The observation of astrophysical neutrinos makes it possible to probe the innermost regions of stars, dense regions from which light cannot escape. Because of their very small interaction probabilities, neutrinos can escape from these inner regions; for the same reason, of course, it is difficult to detect these elusive particles. The motivation for proposing  the first practical solar neutrino experiment was astrophysical, to test directly the hypothesis that stars shine and evolve because of nuclear fusion reactions in their interiors. However, it has become apparent in recent years that solar neutrinos provide a beam of elementary particles that can be used to investigate fundamental physics, in particular to study intrinsic neutrino properties. Of particular importance is the question of whether neutrinos have mass and whether neutrinos transform (oscillate) from one type to another [14-16]. (There are believed to be three types of neutrinos: electron-type νe, muon-type νμ, and tau-type ντ. Only νe are created in the sun, but all types are believed to be emitted from supernovae.) The question of neutrino mass is of fundamental importance for particle physics and for cosmology. In the case of particle physics, neutrino mass may provide a clue to the problem of the origin of the masses of all particles. An interesting class of theories called grand unified theories suggests that all interactions (strong, electromagnetic, and weak) are unified at a large energy scale and that the neutrino masses are inversely proportional to this scale. Two related ways to probe this very high energy scale are the search for proton decay and the search for very small neutrino masses. Our basic ideas about cosmology that explain the microwave background radiation predict a similar background of relic neutrinos. If the heaviest of these neutrinos, usually taken to be ντ, has a mass above a few electron volts,
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